Tamper respondent enclosures are utilised in various applications, including the transport of sensitive information and protecting electronic devices. Examples of various proposals for such enclosures are disclosed in our earlier patent application nos. GB 2 220 513 A, GB 2 258 075 A, GB 2 256 956 A, GB 2 256 957 A, GB 2 256 958 A, and GB 2 270 785 A the disclosures of which are incorporated herein by reference. These enclosures are in the form of envelopes and shallow boxes, the walls of which are formed by folding flexible sheets incorporating tamper detection features. Included in such sheets are layers of flexible material including a matrix of diagonally extended semi-conductive lines printed on thin insulating film. The matrix of lines forms a continuous conductor which is broken if attempts are made to penetrate the film. The circuit is monitored by opening the conductor at one point and measuring the resistance between the two ends of the circuit. Care is required to maintain security at the portions of the sheet which are folded and overlapped, particularly when the enclosure is of wedge-shaped, cuboid or cube form, such as disclosed in GB 2 258 075 A in which a laminate is folded about a plurality of fold lines to form an enclosure. The formation of secure folds and overlapping portions is addressed in 2270785, in which various extensions are provided on a substantially rectangular sheet to assure security at the folded and overlapping portions.
While such arrangements provide secure enclosures, the formation of the enclosures requires great care and thus is relatively time consuming.
Other tamper respondent enclosures and elements are disclosed in GB-A-2 174 830, GB-A-2 108 302, EP-A-0 284 351 and U.S. Pat. No. 5,049,703.
Another form of enclosure is disclosed in U.S. Pat. No. 4,860,351, assigned to IBM Corporation. This enclosure features an electronic circuit card wrapped in a closely spaced coil of insulated conductor such as 35/10,000ths inch nichrome wire. The wire is monitored and breaks in or displacement of the wire due to tampering are detected. The wire-wrapped card is potted in an opaque, cross-linked filled epoxy. The wire is relatively fragile, so attempts to displace it will likely result in breaking the conductor. Also, bypassing a portion of the conductor is difficult because it is difficult to attach to the wire.
Providing a complete wire barrier around a card typically requires one or more kilometers of wire, and the wire winding operation is therefore time consuming and complicated if the wire is to be wrapped in anything other than a very simple pattern. Further, the use of such fragile wire often results in breakages during the winding operation, resulting in high levels of scrapped enclosures.
In addition, the wire barrier comprises four separate strands which are wound simultaneously onto the enclosure, and the outside ends then tied together to form a single enclosure. The tied ends are then tucked beneath the surface of the winding. It would seem likely that this tucking could result in damage to the fragile wire, and also if it was possible to locate and `untuck` the tied ends it might well be possible to unwind the strands.
Australian Patent 598272 (AU-B-74241/87) discloses an electronic memory protection guard which is intended to obviate a number of the difficulties associated with such wound wire barriers. The guard comprises a flexible printed circuit adapted to be folded around the device to be protected. It is suggested that the device and flexible circuit may then be encapsulated in epoxy. In the disclosed example the edges of the printed circuit do not overlap, such that during encapsulation the uncured epoxy could flow between the edges to contaminate the protected device. Also, as the epoxy provides no tamper detection facilities it would be relatively easy to remove areas of epoxy from the printed circuit and then separate the edges of the circuit to gain access to the protected device.
It is among the objects of the present invention to obviate or mitigate these disadvantages.